Some of these roads have a harmonic that just happens to match your truck and trailer and will amplify the up/down motion. I definitely felt it with my last combo. A new truck resolved the issue but it was not trailer weight related. The longer truck wheelbase I believe helped most.
All I can suggest is to try loading the trailer different with more or less weight on the tongue. Or try more or less load in the truck bed.
If you had an older truck I would suggest new shocks. Do inspect yours to verify no loose ends. Maybe just find a different road. That is what I did when I could.